Redmine: #2217

Redmine: #2235

Redmine: #2194

Redmine: #2194

Redmine: #2194
Now it returns GroupItem pointer

Redmine: #2194
In case of SpecularInstrumentItem it can mean switch to non-uniform axis

Redmine: #2194
From now on PointwiseAxisItem is strongly binded to SpecularInstrumentItem
and containes a pointer to it.
Native data is kept intact and translated to required units on demand.

Redmine: #2194

Redmine: #2177

Redmine: #2177

Redmine: #2051

1. InstrumentItem type replacement with domain instrument name
2. using DataItem instead of IntensityDataItem
   in FitSessionController::onPlotUpdate

Redmine: #2051

1. Mask container can now be aquired directly from RealDataItem
2. importMasks and clearMasks methods become virtual in
   InstrumentItems

Redmine: #2051

Redmine: #2051

Redmine: #1974

Redmine: #1974

Redmine: #1974
Also removed unnecessary inclusion in BeamItems.cpp

Redmine: #1974
Changes:
1. BeamItem becomes an abstract base for GISASBeamItem and SpecularBeamItem
2. Distribution reset moved to BeamDistributionItem (see resetToValue)
3. Minor refactoring

Redmine: #1974

Redmine: #1979

Redmine: #1936

Redmine: #1936